1. Field of the Invention
The invention relates to a ceramic material of low volume resistivity, and a member for the production of semiconductors using the material.
2. Related Art Statement
An electrostatic chuck system utilizing Johnson-Rahbek effect is useful for the adsorption and retention of semiconductor wafers. In such system, the volume resistivity of the substrate of the chuck may preferably be 108 to 1013 xcexa9xc2x7cm for improved adsorption force and speed of response. It is therefore desirable to control the volume resistivity of the substrate within 108 to 1013 xcexa9xc2x7cm in a temperature range intended for use, in the development of an electrostatic chuck.
For example, the applicant filed a Japanese patent publication (Kokai) with a laid-open number 315867/1997 and disclosed that the volume resistivity of aluminum nitride with a high purity may be adjusted to 108 to 1013 xcexa9xc2x7cm at room temperature by adding a small amount of yttrium oxide thereto.
Japanese patent publication (Kokoku) with a publication number 46032/1988 discloses a process for producing an aluminum nitride sintered body. According to the claim 1, aluminum nitride powder containing 1 weight percent of oxygen is mixed with 0.01 to 15 weight percent of the oxide of a metal element selected among yttrium, lanthanum, praseodymium, niobium, samarium, gadolinium and dysprosium to obtain mixed powder. The powder is then shaped and sintered to obtain an aluminum nitride sintered body having a high thermal conductivity and containing 0.01 to 20 weight percent of oxygen. According to xe2x80x9cexample 1xe2x80x9d in the publication, aluminum nitride powder (with a mean particle diameter of 1 xcexcm) containing 1 weight percent of oxygen is mixed with 3 weight percent of samarium oxide to obtain mixed powder. The powder is then subjected to hot press at a pressure of 300 kg/cm2 and a temperature of 1800xc2x0 C. for 1 hour to obtain a sintered body with a heat conductivity of 121 Wxc2x7m/k at room temperature.
The effects of the addition of a rare earth element other than yttrium into an aluminum nitride sintered body on its volume resistivity was not studied, in the Japanese patent publication (Kokai) with a laid-open number 315867/1997. In the Japanese patent publication (Kokoku) with a publication number 46032/1988, a rare earth element is added to raw powder of aluminum nitride for improving the thermal conductivity of the resultant aluminum nitride sintered body. The effect of the addition on the volume resistivity of the sintered body was not studied.
An object of the invention is to provide a material, composed of an aluminum nitride sintered body, having a low volume resistivity of not higher than 1xc3x971013 xcexa9xc2x7cm at room temperature.
Another object of the invention is to provide an aluminum nitride sintered body having a low resistivity at room temperature.
Still another object of the invention is to provide a member used for producing semiconductors, in which the volume resistivity may be controlled depending on the application, and to prevent the contamination of semiconductors when the member is exposed to a corrosive gas.
The invention provides a material having a volume resistivity at room temperature of not higher than 1xc3x971013 xcexa9xc2x7cm (more preferably not higher than 1xc3x971012 xcexa9xc2x7cm). The material is composed of an aluminum nitride sintered body containing samarium in a converted content calculated as samarium oxide of not lower than 0.04 mole percent. The sintered body contains aluminum nitride phase and samarium-aluminum complex oxide phase.
The invention further provides an aluminum nitride sintered body containing samarium in a converted content calculated as samarium oxide of not lower than 0.04 mole percent. The sintered body contains aluminum nitride phase and SmAl11O18 phase.
The content of aluminum in the aluminum nitride sintered body should be enough for forming aluminum nitride particles as the main phase. The content may preferably be not lower than 35 weight percent, and more preferably be not lower than 50 weight percent, of the sintered body.
The inventors have found that the volume resistivity of an aluminum nitride sintered body at room temperature may be reduced to 1xc3x971013 xcexa9xc2x7cm or lower, by increasing the content of samarium as converted content calculated as the oxide to 0.04 mole percent or more and by forming aluminum nitride phase as well as samarium-aluminum complex oxide phase. Such effect of the addition of samarium on the reduction of the volume resistivity of an aluminum nitride sintered body has not been known.
Further, the inventors have studied an aluminum nitride sintered body with the reduced volume resistivity (especially the microstructure and composition) in detail. Then they found that the volume resistivity is considerably reduced when at least SmA111O18 phase is formed in the sintered body.
The inventors have further studied the microstructure of an aluminum nitride sintered body with the reduced volume resistivity using various kinds of apparatuses described below, and found the following characteristic microstructure. Such sintered body has network microstructure made of samarium-aluminum complex oxide phase. The oxide phase is continuously formed and surrounds aluminum nitride grains in the body. It is considered that such network microstructure may contribute to the reduction of the volume resistivity.
According to the Japanese patent publication (Kokoku) with a publication number 46032/1988, 3 weight percent of samarium oxide (converted content calculated as samarium) is added to raw powder of aluminum nitride to provide mixed powder, which is then hot pressed to obtain an aluminum nitride sintered body. The publication, however, does not teach the change of the volume resistivity of the sintered body. Moreover, the publication does not teach the relationship between the precipitation of SmAl11O18 phase and the volume resistivity, as well as the network microstructure formed in the sintered body. It is considered that the discovery of the relationship and microstructure was beyond the state of art at that time.
The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.